1. Field of the Invention
The present invention relates to a transfer method for secret keys to be used in encrypted communication, and especially relates to a technique for specifying one out of a plurality of distributed secret keys that is to be used in encrypted communication.
2. Description of the Prior Art
In recent years, secret key encrypted communication systems have been developed which encrypt data for using a secret key and transmit the encrypted data using a transfer medium or record the encrypted data onto a recording medium for later reproduction. In such systems, the transmission apparatus and the reception apparatus respectively perform encryption and decryption using a shared secret key which has been provided to both apparatuses beforehand.
When both the transmission apparatus and the reception apparatus are provided with a plurality of secret keys, it is necessary to specify one of these secret keys before encrypted communication is commenced so that both apparatuses will use the correct key. This is performed to ensure that the reception apparatus will be able to decrypt the cryptograms transmitted by the transmission apparatus.
In conventional encrypted communication systems, however, should the secret key be obtained by an unauthorized third party, it will be necessary to replace the secret key with a new secret key in all of the communication devices which previously used this secret key. When the secret key is permanently stored in the components of the communication apparatuses, such as in a ROM (Read Only Memory), or when the secret key was used by a large number of communication apparatuses, this means that a great deal of work needs to be performed to establish the new key.